Application, research and industrial production of aluminide-based alloys and titanium metal matrix composites (TiMMC) have progressed over the last 15 to 20 years to the current status of a pre-production engineered material. During that time, numerous industrially manufactured articles have been fabricated and tested with encouraging results. TiMMCs have demonstrated to be useful in a wide variety of aerospace applications for airframes and propulsion systems. The remaining challenge is the manufacturing development of titanium aluminide alloys, TiMMC and TiMMC-reinforced components that simultaneously satisfy the market-driven requirements of affordability, performance and reliability.
Most of these materials are based on or contain reactive metals and alloys initially supplied in powder form because powder metallurgy is the most effective way to process metal aluminide-containing alloys and composites. Reactive alloys might be determined as alloys that exhibit an increase in chemical interaction with oxygen, nitrogen, carbon, etc. at elevated temperatures. Titanium aluminide, high strength titanium alloys, nickel aluminides, zirconium aluminides, iron aluminides, beryllium alloys, refractory metals, niobium, and other metals represent this group of such reactive alloys. Dense sheets or foils of reactive metals such as titanium aluminides or nickel aluminides are used for manufacturing important structural elements ideally designed for aircraft and space applications, where high service temperature and high strength-to-density components are required. However, the fabrication of such products as thin gauge gamma-titanium aluminide sheets and shaped articles is extremely difficult because of their inherent low ductility. In addition, oxidation of these alloys is drastically increased at elevated temperatures that significantly hinder hot forming of sheet. Also, the undesired diffusion of a gas into a metal surface produces a decrease in ductility.
The aerospace industry continues to strive for larger production yields while reducing production costs, providing processing stability, and increasing the uniformity of microstructure of single-phase or multi-component titanium aluminide alloys, as well as composite materials based on these alloys.
The need for elevated temperatures during reactive metal processing has produced a number of previous techniques, which eliminate oxidation atmospheres from the environment of the metal during high-temperature processing. For example, hot working in large vacuum chambers or in inert gas environments is a common technique. However, the costly manufacturing facilities, which are required in these processes, add additional expenses to the final product. In many applications, an oxide layer is removed from a metal section by machining or the like.
Many technologies, known for manufacturing sheets of reactive metals, incorporate special coatings, claddings, or capsules that protect the reactive metal workpieces from oxidation and degradation during the hot forming process. For instance, in U.S. Pat. No. 3,164,884 to Noble et al., a method for the multiple hot rolling of sheets is disclosed, in which cover plates and sidebars are assembled around inner reactive metal plates separated by a release agent. The sidebars are welded to the cover plates and to each other along their outer edges. The release (separating) agents are liquid mixtures of aluminum, chromium, or magnesium oxides. Additional built-in vent holes permit gases, which are formed in the package during the hot rolling process, to escape.
In U.S. Pat. No. 5,121,535 to Wittenauer et al., a method of forming a reactive metal workpiece was created, which is protected from high-temperature oxidation during hot working by placing the workpiece in a malleable metal enclosure with a film of release agents interposed between major mating surfaces of the reactive metal section and the metal jacket. In a preferred embodiment, a metal section of a reactive metal is placed in a non-reactive metal frame. The reactive metal section and frame are then interposed between non-reactive metals from the top and bottom plates, with a release agent that exhibits viscous, glass-like properties at high temperatures being disposed at the interfaces of the reactive metal sections. The assembly is then welded together near the perimeter so that the release agent is sealed between the sections.
The welded assembly may then be hot rolled under pressure to the desired gauge using conventional hot rolling machinery and procedures to form the sheet. Other hot working techniques may be employed where suitable. Thus, accelerated oxidation during the high-temperature hot working of the reactive metal section is prevented using this patent, by encapsulating the reactive metal section in a non-reactive metal jacket.
Thereafter, the formed assembly or laminate is cooled, and the rolled assembly is sheared to remove the welded edges. The non-reactive metal sections are simply peeled from the reactive metal core by virtue of the brittle, non-cohesive release agent.
W. J. Truckner and J. F. Edd (U.S. Pat. No. 5,405,571) proposed a combination of tape casting and consolidation by hot pressing to manufacture thin sections from powders of titanium alloys, titanium aluminides, nickel aluminides, and molybdenum disilicide. The main drawback of this method is the residual porosity that is present in the final alloy due to traces of the polymer binder used in tape casting.
The U.S. Pat. No. 5,863,398 provides the manufacture of reactive alloys by hot pressing followed with sintering under pressure of 3000-5000 psi at 1300-1500° C. The method is characterized by low productivity and density gradient along the resulting thin material. This density gradient is caused by an error in parallelism between the punch and matrix of the hot pressing die that exists in the procedure.
K. Shibue, with co-workers, reported on the manufacture of shaped TiAl alloy by cold extrusion of an elemental powder blend in an aluminum can followed by hot isostatic pressing (U.S. Pat. No. 5,372,663). This method can be used to produce only symmetrical articles, e.g., rod-like. It is not suitable for thin sheets or strips.
Some cellular metal materials have been extensively developed and investigated in recent years. The potential for applying metal foams in lightweight constructions is the stiffness and impact absorption (see review of J. C. Benedyk, Light Metal Age, 2002, 60(3,4), p. 24-29). These foams can be processed by cold or hot deformation, as this was made with aluminum foam in the U.S. Pat. No. 5,972,285, to obtain controlled structure and porosity. These foams were also used as a structural component of composite materials being infiltrated with molten metals or filled with ceramic powders.
For example, U.S. Pat. No. 6,080,219 discloses composite materials consisting of nickel or ceramic foam filled with metal or plastic powders to obtain filters having controlled porosity. Such composites cannot be considered as reliable structural materials because their strength is completely dependent upon the properties of the foam, which are always lower than mechanical properties of a solid metal.
A porous iron foam structure is infiltrated by molten magnesium to produce a composite material with solidified Mg matrix filling the voids of the foam, as disclosed by Lev Tuchnsky in the U.S. Pat. No. 6,254,998. This composite has poor corrosion resistance because of the very low Fe content, which reduces the corrosion resistance of magnesium drastically. But more importantly, the Fe—Mg composites have no reserves to improve their physical or mechanical properties due to very low solubility of both elements.
Aluminum foam reinforced with steel wires (U.S. Pat. No. 3,941,182) is a more promising composite than the foam-based materials mentioned above, but it is not suitable as a structural material for heat-resistant and high-loaded applications.
All previous technologies of fabricating thin dense sheets and shaped articles from reactive alloys have considerable drawbacks, which make them undesirable in terms of strength and ductility of resulting titanium aluminide articles, sufficient protection from oxidation, cost, and production capacity, especially if these articles were produced initially from reactive alloy powders, which require additional hot working cycles for compacting. The resulting porosity causes very rapid oxidation of the reactive alloy to a substantial depth, and capsules designed in known inventions do not fully protect the sintered section from rapid oxidation. A significant difference in structural and mechanical properties between sintered sheets, produced from reactive metal powder, and the frame (capsule), produced from non-reactive wrought metal, result in non-uniform deformation and stress concentration of the laminate package during the hot rolling process. Cracks occur in various areas of the sintered section during the first cycles of hot rolling and do not allow it to maintain a stable manufacturing process.
Cellular metals were used in composites only as stiff structural component perceiving mechanical loading and protecting soft matrix. In this fashion, the strength of the composite is governed by the strength of the metal foam, e.g., by the strength of aluminum foam that is usually insufficient. Even the strength of such composites based on iron or nickel foams is significantly lower than the strength of solid metals due to the high-volume porosity of the foam.
Therefore, it would be desirable to provide (a) a high-strength and fully-dense metal matrix composite based on prospective aluminide alloys, and (b) a cost-effective method of producing this composite using powder reactive alloys, which improves the mechanical performance of resulting materials, and eliminates destructive oxidation during high-temperature processing. The present invention achieves this goal by using an aluminum skeletal structure filled with a reactive metal powder prior to hot working, and by providing a method by which the pre-structured skeleton/powder composite can be formed into fully-dense sheets or shaped articles in a hot working process combining loose sintering, hot axial pressing, hot isostatic pressing, and/or longitudinal hot deformation followed by a specified heat treatment.